Preparation of alkenyl aromatic monomer butadiene rubber and preparation of impact resistant resin therefrom

ABSTRACT

Butadiene is partially polymerized anionically in the presence of styrene to form a styrene-butadiene rubber of low styrene content. The resultant living polymer is terminated, butadiene removed, and optionally the styrene polymerized with or without additional styrene to form impact resistant styrene polymer.

This application is a continuation-in-part of the copending applicationSer. No. 058,704, filed July 18, 1979, now abandoned.

Solution polymerized butadiene polymers such as polybutadiene,styrene-butadiene polymer and like rubbery butadiene polymers preparedwith a lithium based catalyst have been recognized as having highlydesirable physical properties for a number of applications. Thepolymerization of butadiene polymers with lithium based catalyst isdisclosed in U.S. Pat. No. 3,317,918, the teachings of which areincorporated by reference thereto. One end use for such butadienepolymer is in the reinforcement of styrene polymers and the preparationof impact resistant rubber reinforced molding and extrusioncompositions. U.S. Pat. Nos. 3,264,374 and 3,976,721 disclose butadienepolymer reinforced polystyrene. Such butadiene polymers have beenprepared by a variety of methods. However, for many purposes the lithiumbased catalyst provides a preferred product. The block polymerization of1,3-butadiene is disclosed in U.S. Pat. No. 3,970,607 which employs a1,2-diene as a reaction regulator to prevent excessive exotherm per unitof time during polymerization. Oftentimes such lithium initiated orsolution polymerized rubbery butadiene polymers are available in theform of a solidlike rubbery mass or an extremely viscous liquid. In thepreparation of a rubber reinforced alkenyl aromatic polymer such aspolystyrene, they are dissolved in the styrene monomer and the resultantrubber solution with or without diluent, subsequently polymerized eitherby the mass polymerization technique or mass-suspension polymerizationtechnique to give the desired impact resistant styrene polymer. Thepreparation of the alkenyl aromatic monomer-rubber solution requiressubstantial equipment, labor and elapsed time. Oftentimes the rubber isprovided in the form of bales. The bales must be unwrapped and oftengreat care taken that none of the wrapping material remains on the bale.The bale of rubber is then fed to a chopping device which severs thebale into a plurality of irregular sticky particles which are deliveredto a body of alkenyl aromatic monomer such as styrene, which is agitateduntil, at least hopefully, a uniform solution of rubber in monomer isobtained. The monomer-rubber solution is oftentimes filtered andsubsequently subjected to polymerization conditions.

It would be desirable if there were available an improved method for thepreparation of rubbery butadiene polymer.

It would be desirable also if there were available an improved processfor the preparation of alkenyl aromatic monomer-butadiene containingrubber solutions using alkenyl aromatic monomer as solvent.

It would be advantageous if such a process would not require handling ofundissolved rubber.

These benefits and other advantages are achieved in a process for thepreparation of an alkenyl aromatic butadiene rubbery polymer, the stepsof the method comprising providing a solution comprising 80 to 30 partsby weight of alkenyl aromatic monomer, 20 to 70 parts by weight ofbutadiene (advantageously from 75 to 35 parts by weight alkenyl aromaticmonomer and 30 to 60 parts by weight butadiene), and up to 50 weightpercent based on the total weight of the solution of a solvent which isgenerally inert under conditions of polymerization, initiatingpolymerization with a lithium based polymerization initiator until about5 to about 30 weight percent of the combined weight of alkenyl aromaticmonomer and butadiene has been converted to form a rubbery alkenylaromatic monomer-butadiene polymer, terminating the polymerization andseparating unreacted butadiene therefrom.

In an advantageous embodiment of the invention polymerization is againinitiated to induce polymerization of alkenyl aromatic monomer, oralkenyl aromatic monomer with a comonomer such as acrylonitrile, untilthe desired quantity of the alkenyl aromatic monomer, or alkenylaromatic monomer and comonomer has been converted to alkenyl aromaticpolymer, and subsequently isolating from the reaction mixture an alkenylaromatic polymer reinforced with a rubbery alkenyl aromatic monomerbutadiene polymer.

By the term alkenyl aromatic monomer is meant an alkenyl aromaticcompound having the general formula ##STR1## wherein R₁ is hydrogen orthe methyl radical and R₂, R₃, R₄ are selected from the group consistingof hydrogen and alkyl radicals containing up to 10 carbon atoms with thefurther limitation that the total number of carbon atoms in R₂, R₃, andR₄ is not greater than 10. Examples of such monovinyl alkenyl aromaticmonomers are styrene, vinyl toluene (all isomers) the paraisomer beingpreferred, α-methylstyrene, paratertiary butyl styrene, 3,4-dimethylstyrene, 2-ethylhexyl styrene (all isomers) n-decyl styrene (allisomers), n-butyl styrene.

Butadiene and alkenyl aromatic monomer used in the present inventionshould be generally free of active hydrogen compounds which woulddeactivate a lithium based catalyst.

Lithium based catalysts suitable for the practice of the presentinvention are set forth at length in U.S. Pat. No. 3,317,918, theteachings of which are herewith incorporated by reference thereto.Generally, from a standpoint of availability and convenience ofhandling, n-butyllithium is preferred.

Polymerization of the rubbery alkenyl aromatic butadiene polymer inaccordance with the present invention is advantageously conducted at atemperature between about 10° C. and 70° C. and preferably 30° C. to 50°C. Desirably such polymerization is conducted in a vessel equipped withan agitator and a reflux condenser which permits boiling of thebutadiene to act as a heat transfer agent.

On polymerization in accordance with the present invention, it isessential that only relatively low conversion of the butadiene torubbery polymer be permitted. The kinetics of polymerization are suchthat if the major portion of the butadiene is converted to polymer, thepolymer is no longer rubbery because of excessive alkenyl aromaticmonomer content. If the conversion of butadiene exceeds about 30 percentby weight of the butadiene, the possibility of thermal runaway, that is,uncontrolled rapid polymerization of the alkenyl aromatic monomerbecomes very great. As the conversion of butadiene to polymer increases,so does the percentage of alkenyl aromatic monomer in the polymerincrease, and the rubbery characteristics desired for reinforcement arelost. Termination of the lithium induced polymerization is readilyaccomplished by the addition of compounds which will donate a proton.Typically terminating compounds include water, methyl alcohol, ethylalcohol, propyl alcohol, acetic acid, propionic acid and the like. Oncethe anionic polymerization has been terminated, desirably butadiene isremoved from the system, generally by distillation with or without theuse of subatmospheric pressure. The butadiene is readily condensed andmay be used again. After the removal of the butadiene, free radicalpolymerization of the alkenyl aromatic monomer with agitation can bestarted either by the use of conventional free radical initiators suchas peroxy compounds, azo compounds or combination of peroxy and azo, oralternatively free radical polymerization can be initiated thermally.

Butadiene rubbers prepared in accordance with the present invention havemolecular weights from 30,000 grams per mole to about 700,000 grams permole and contain from about 1 to 40 weight percent alkenyl aromaticmonomer copolymerized therein. Advantageously, the rubbers contain fromabout 2 to 25 weight percent alkenyl aromatic monomer and preferablyfrom 5 to 15 weight percent styrene when being used for reinforcing astyrene polymer matrix such as polystyrene or styrene-acrylonitrilecopolymer.

Generally, the free radical polymerization is conducted at a temperatureof from about 60° C. to about 170° C. Peroxy initiated free radicalpolymerization is generally conducted in the range of 60° C. to about170° C. while thermal initiation is generally employed in thetemperature range of from about 110° C. to about 170° C. During at leastthe initial portion of the polymerization of alkenyl aromatic monomer,agitation is desirable in order to obtain the preferred impact resistantpolymers. Suitable equipment for such polymerization is set forth inU.S. Pat. No. 3,243,481, the teaching of which is herewith incorporatedby reference thereto.

When polymerization of the alkenyl aromatic monomer has occurred to thedesired degree, residual monomer is removed by heating the reactionmixture to a temperature of from about 180° C. to about 250° C. under apressure of from about 0.10 to 100 millimeters of mercury and exposingas much surface of the reaction mixture as possible in a devolatilizingchamber and subsequently cooling the polymer. Such polymer is generallyuseful for extrusion and injection molding.

The invention is further illustrated but not limited by the followingexamples:

EXAMPLE 1

A one-liter round bottom flask is equipped with a dry ice cooled refluxcondenser and an agitator. The vessel is provided with a nitrogenatmosphere and charged with 318 grams of purified styrene and 170 gramsof purified butadiene. The contents of the flask were at ambienttemperature (about 22° C.). The polymerization was initiated by theaddition of 2 milliliters of a 0.523 normal solution of n-butyllithiumin hexane. The polymerization temperature in the flask varied from about14° C. to 24° C. and was governed by the refluxing rate of the butadienefrom the reaction mixture. The polymerization was terminated 5 hoursafter the addition of the n-butyllithium by the addition of about 0.2milliliters of n-propanol. The polymer was recovered by precipitationwith methanol to yield 45.5 grams of a rubbery styrene-butadienepolymer. The yield was 9.3 weight percent based on the weight of theinitial monomers. Gel permeation chromatography was employed todetermine the molecular weight using both an ultraviolet and refractiveindex detectors. The molecular weight was 187,000 grams per mole and thepolymer contained 14.9 percent styrene and 85.1 percent butadiene,percentages being weight percentages. Employing the reactivity ratiosreported by Hsieh and Glaze (Rubber Chem. Tech., 43, 22, 1970), whereinthe reactivity ratio for styrene was taken at 0.1 and that for butadieneas 12.5, the calculated composition was 14.5 percent styrene and 85.5percent butadiene.

EXAMPLE 2

A two-liter reactor with agitator was flushed with nitrogen and chargedwith 554 grams of purified styrene and 677 grams of purified butadiene.Polymerization was initiated with 12.5 milliliters of 0.55 normaln-butyllithium in hexane solution. The reaction mixture was heated to45° C. and heating discontinued. Dry ice was placed on the top of thereactor to condense butadiene vapors and thereby maintain thetemperature of the contents below 50° C. One hour and 45 minutes afterthe addition of the n-butyllithium, polymerization was terminated by theaddition of 5 milliliters of one normal ethyl-benzene solution ofn-propanol. The reaction mixture was sampled and had a solids content of21.6 weight percent. The molecular weight was determined using theapparatus of Example 1. The molecular weight was 322,000 grams per mole.The polymer contained 6.8 weight percent styrene, the remainder beingbutadiene. The reaction mixture was then admixed with 2 liters ofstyrene and the excess butadiene monomer removed by agitating thesolution and applying vacuum thereto. 1123 Grams of the mixtureconsisting of 84 grams of styrene-butadiene rubbery polymer and 1039grams of styrene was further diluted with an additional 228.5 grams ofstyrene. To this mixture was added 150 grams of ethylbenzene, 3.75 gramsof mineral oil, 2.25 grams of stabilizer commercially available underthe trade designation of Irganox 1076, 1.05 grams of alphamethylstyrenedimer and 3 grams of a 25 weight percent active solution of1,1-di(tertiarybutylperoxy)cyclohexane in ethylbenzene. 1200 Grams ofthe mixture was added to an agitated batch polymerization reactorwherein the temperature was raised from 110° C. to 160° C. over a periodof 7 hours. After 4 hours an additional 200 grams of the feed mixturewas added to the reactor. At the end of 7 hours heating wasdiscontinued, the solids of the mixture were 72.1 weight percent. Themixture was poured into shallow pans and placed in a vacuum oven at atemperature of about 200° C. for a period of 90 minutes. Thedevolatilized polymer was removed from the shallow pans, ground to forma plurality of particles which were subsequently compression molded toprovide samples for physical property determinations. The tensilestrength at yield of the polymer was 2840 pounds per square inch;tensile strength at break 2965 pounds per square inch. The exampleshowed 28.1 percent elongation at break. The notched Izod impactstrength was 1.4 foot-pounds per inch of notch and the Vicat heatdistortion temperature was 212° F. which are typical properties forconventional impact resistant polystyrene of like rubber content.

EXAMPLE 3

A plurality of rubbers were prepared employing the general procedure ofExample 1 wherein the styrene-butadiene monomer ratio by weight was65:35, varying quantities of toluene were employed as a diluent and theinitial temperature and n-butyllithium concentration was varied as wellas polymerization time. The results are set forth in Table I.

                                      TABLE I                                     __________________________________________________________________________             Effective                                                                            Initial                                                                           Max.                                                                              Length      Copolymer                                 %        n-BuLi Temp                                                                              Temp                                                                              of Runs                                                                            Conv.                                                                             M.sub.w x                                                                        % Styrene                                 Run #                                                                             Toluene                                                                            Conc. mmol/l                                                                         °C.                                                                        °C.                                                                        Hrs. %   10.sup.-3                                                                        Cal.                                                                             Obs.                                   __________________________________________________________________________    3   50   0.39   40  --  11/4 24.7                                                                              307                                                                              18.1                                                                             18.9                                   4   22   0.76   23  38  31/2 19.4                                                                              195                                                                              16.5                                                                             14.9                                   5   22   0.54   23  45  3    13.0                                                                              166                                                                              15.2                                                                             12.7                                   6    0   0.77   16  25  41/2 11.8                                                                              142                                                                              15.0                                                                             12.2                                   7   10   0.24   18  28  51/2  7.7                                                                              299                                                                              14.3                                                                             15.7                                   __________________________________________________________________________     n-BuLi = normal butyllithium                                                  Temp = temperature                                                            °C. = degrees Centigrade                                               Hrs. = hours                                                                  Conv. = conversion                                                            M.sub.w = weight average molecular weight grams per mole                      Sty. = styrene                                                                Cal. = calculated                                                             Obs. = observed                                                          

In a manner similar to the foregoing examples, beneficial results areobtained when the styrene is replaced wholly or in part by one or moreof the alkenyl aromatic monomers hereinbefore set forth.

As is apparent from the foregoing specification, the present inventionis susceptible of being embodied with various alterations andmodifications which may differ particularly from those that have beendescribed in the preceding specification and description. For thisreason, it is to be fully understood that all of the foregoing isintended to be merely illustrative and is not to be construed orinterpreted as being restrictive or otherwise limiting of the presentinvention, excepting as it is set forth and defined in thehereto-appended claims.

What is claimed is:
 1. A process for the preparation of an alkenylaromatic-butadiene polymer, the steps of the method comprising providinga solution comprising 80 to 30 parts by weight of alkenyl aromaticmonomer 20 to 70 parts by weight of butadiene, and up to 50 weightpercent based on the total weight of the solution of a solvent which isgenerally inert under conditions of polymerization, initiatingpolymerization with a lithium based polymerization initiator at atemperature of from about 10° to 70° centigrade until about 5 to about30 weight percent of the combined weight of alkenyl aromatic monomer andbutadiene has been converted to form a rubbery alkenyl aromaticmonomer-butadiene polymer containing from about 2 to 25 weight percentalkenyl aromatic monomer, terminating the polymerization and separatingunreacted butadiene therefrom and initiating free radical polymerizationof the alkenyl aromatic monomer present.
 2. The process of claim 1wherein the lithium based polymerization initiator is a butyllithium. 3.The method of claim 1 wherein polymerization of butadiene is initiatedat a temperature of from about 30° C. to about 50° C.
 4. The method ofclaim 1 wherein at least a portion of heat generated by polymerizationof the butadiene is removed by refluxing butadiene.
 5. The process ofclaim 1 wherein the alkenyl aromatic monomer is styrene.
 6. The processof claim 4 including the step of adding styrene to the reaction mixtureafter the polymerization has been terminated.
 7. The method of claim 6including adding styrene prior to initiating free radical polymerizationof the styrene.
 8. A process for the preparation of a styrene-butadienepolymer, the steps of the method comprising providing a solutioncomprising 80 to 30 parts by weight of styrene, 20 to 70 parts by weightof butadiene and up to 50 weight percent based on the total weight ofthe solution of a solvent which is generally inert under conditions ofpolymerization initiating polymerization with a lithium basedpolymerization initiator until about 5 to about 30 weight percent of thecombined weight of styrene and butadiene has been converted to from arubbery styrene-butadiene polymer, terminating the polymerization andseparating unreacted butadiene therefrom, subsequently initiatingpolymerization to induce polymerization of styrene until a desiredquantity of styrene has been converted to styrene polymer andsubsequently isolating from the reaction mixture styrene polymerreinforced with a rubbery styrene-butadiene polymer.